This invention is in the field of photoflash lamps of the so-called high-voltage type, which contain a primer material connected across inlead wires, there being no conventional filament in such a lamp. These lamps further contain a hermetically sealed light-transmitting envelope made of the glass at the present time along with a combustion-supporting gas such as oxygen together with a loosely distributed filling of a suitable light-producing combustible material such as shredded foil of zirconium, aluminum or hafnium, for example, which upon ignition produces a high intensity flash of actinic light.
Electrically actuated photoflash lamps may be classified generally into two voltage types: low-voltage and high-voltage. The low voltage types usually are intended to be flashed by a battery, or a charged capacitor, having a voltage of about 1.5 volts to 15 volts, whereas the high-voltage flashlamps are intended to be flashed by a firing pulse of a few hundred volts or greater such as can be produced by striking a piezoelectric material. Conventional low-voltage flashlamps contain a filament connected across inlead wires. When the filament is heated by a firing current, it ignites a primer material which in turn ignites a combustible material such as metal foil which, with the aid of oxygen in the lamp, produces a flash of light.
In typical high-voltage flashlamp constructions, the primer material is connected directly across and between a pair of inlead wires extending into the lamp envelope. The primer material may be positioned and carried in the lamp on top of a glass or ceramic insulating member through which the inlead wires extend, or may be carried in a cavity in such a member. In another construction, the primer material is carried on or in a depression in the inner wall of the envelope at the bottom of the lamp. In another high-voltage flashlamp construction, disclosed in U.S. Pat. Nos. 2,868,003 and 3,000,200, both to Warren Albrecht, the primer material is applied to one or both of the inlead wires within the lamp and the electrical circuit is completed through the combustible shredded metal foil in the lamp.
Various primer materials suitable for the ignition means in the above type lamp constructions are known. The known primer materials generally consist of a solid mixture of a readily combustible fuel such as phosphorous with an oxidizer compound for the fuel such as alkali metal and alkaline earth metal chlorates, as well as perchlorates including sodium perchlorate, potassium perchlorate, barium chlorate, sodium chlorate, and potassium chlorate. The fuel in the primer material is generally a powdered combustible incandescible metal such as zirconium, hafnium, thorium, aluminum, magnesium, boron, silicon or their alloys which upon actuation by a high voltage pulse ignite the filamentary combustible material. The known primer materials are generally prepared as liquid suspensions in an organic or aqueous solvent to provide an adherent mass of the primer material between the spaced apart inleads. A known organic liquid suspension for the primer material, that is, described in U.S. Pat. No. 2,972,937 to C. G. Suits, utilizes nitrocellulose as the binder agent and it requires careful handling during storage and lamp manufacture to avoid accidental ignition. A less sensitive aqueous suspension of the primer material utilizing the conventional ignition mixture employs hydroxyethylcellulose or methoxycellulose, polyvinyl pyrrolidone as water-soluble binder agents.
A number of considerations are important for the primer material to reliably ignite the combustible material in a high-voltage flashlamp of the type above generally described so that the desired light output is obtained in a relatively short time period. More particularly, the primer material must be sensitive enough in oxygen or another combustion-supporting gas to reliably ignite the principal filamentary combustible material distributed within the lamp glass envelope with a blast of sparks in order for the principal combustion reaction to take place whereby the light output from the lamp is produced at a desired light level and time span. If the blast velocity of the primer material is excessive, then the filamentary combustible material becomes packed in the lamp envelope with subsequent reduction/in light output or slower burning rate. In certain high-voltage lamp constructions intended for use in the multilamp photoflash array where each individual lamp desirably provides an "open circuit" to successive high-voltage pulses after flashing, it is also important that the blast characteristics of the primer material as well as the principal combustion reaction not produce a significantly low resistance short between the spaced apart inleads of the lamp. A still further important operating characteristic of the primer material in a high-voltage flashlamp is the voltage at which the primer's electrical resistance breaks down or decreases thus allowing sufficient energy to be transferred from the firing pulse at its available voltage level to cause primer ignition. Said in another way, excessive variation in this breakdown voltage characteristic could lead to a failure of the lamp to flash or to flash an inferior light output. The conventional primer mixtures have been found not to reliably provide proper ignition in a high-voltage type flashlamp which can be attributed at least in part to incomplete combustion of the combustible fuel constituent in the primer material. While excess stoichiometric amounts of the oxidizing constituent for said combustible fuel can be employed as a means to provide a complete fuel combustion, such compensation leads to producing a general oversensitivity in the primer material accompanied by an overly vigorous blast characteristic.